LH6 5.3L Engine Upgrade Guide: Expert Advice for LH6 Engine Mods to Maximize Performance

Intro to the LH6 Engines

The LH6 came in GM trucks and SUVs in 2005-09 and made 315 horsepower right out of the box.

It’s a Gen. 4 5.3L aluminum-block engine, so you’re getting the better intake manifold, connecting rods, and ECM. The engine also had 799 or 243 casting heads which are closely related to the LS6.

It was a crossover engine which meant it shared the old Gen. 3’s 24X reluctor crank and plastic wedge-style chain damper in 2005-06 before going to 58X and a spring loaded tensioner in 2007

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For LH6 engine specs, read this:

• LH6 5.3L Engine Specs: Performance, Bore & Stroke, Cylinder Heads, Cam Specs & More

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How to Get More Power From Your LH6

These engines start life in trucks and SUVs.

The LH6 is gaining popularity for a couple reasons:

• It’s an aluminum block alternative to 5.7L engines that are getting more difficult to find. The intake and accessory drive swap over, but a Lingenfelter crank sensor trigger conversion is required on the 2007 and later engines.
• General Motors made a bazillion of them which helps keep prices down.

Do they have a downside? Just one. Most will want to get rid of the AFM immediately.

Fortunately, deleting one or both is straightforward and easy.

Basic Bolt-On Upgrades for LH6 Engines

Trucks are the everyday hotrods of the 21^st century and you can think of the LH6 as the modern equivalent of the old 327—a long-time favorite of hot rod builders.

For many people, it starts off with a cold air intake and aftermarket exhaust. The problem is that the engine starts to sound REALLY good and owners find themselves wanting to go faster. All these can be done in the garage, but the tune won’t be optimized.

At this point, we recommend talking to your chassis dyno tuner and decide on a computer programmer. Whether you’re towing on low-octane or springing for good fuel, a tuner can dial the ECM and take it to the next level.

The 5.3L loves to rev, so raising the rev limit and shift points is common. It also makes it easier to tune with a bigger cam and injectors later on. Before you go, make sure to install a colder thermostat to open up the tuning window.

Below are more upgrades that will improve the performance of the LH6 engines.

Upgrading the LH6 Camshaft and Valvetrain

As we mentioned earlier, the engine came with AFM—which is known to fail.

If your engine is in good running condition now, you can install an AFM disabler.

If a lifter is already failing, it’s best to go with an AFM delete kit.

The earlier trucks may also have high oil consumption, but there’s a fix for that too. In October 2010, GM installed a deflector on the oil pressure relief valve in the oil pan.

And if you’re in this deep, why not put a cam in it?!

Generally, 3-bolt cams are more common and just requires the addition of a 3-bolt 4-pole upper gear to convert it over.

So how do you choose a cam?

The price point of a LS3 or LS9 cam is good, but they make the engines pretty soggy below 5,000 rpm. This is due to the intake valve closing (IVC) point being late and bleeding off a lot of compression down low.

What you want is a dedicated truck cam.

What’s the difference?

To maximize torque in the mid-range, manufactures close the intake valve at about 40 degrees (@.050 in.) after bottom dead center and alter the intake valve opening to set the idle quality. For a good idle and low end, you’ll often see the 5.3L truck cams in the 204-215 @ .050 range if it’s still a daily driver.

If the engine is being swapped into a lightweight car with deep gears, converter, etc., the LH6 responds well to bigger cams.

If you’re planning to turbocharge your 5.3L, you’ll want a dedicated turbo cam.

They reduce overlap to keep high turbine inlet (backpressure) from flowing back into the cylinder. Generally, supercharger cams and nitrous cams will have slightly more lobe separation and longer exhaust duration.

Drop-in .500 in. lift cams are popular, but LS6 springs allow you to run .550 in. lift and extend the rpm range. Spring life isn’t a problem because trucks generally don’t spend a lot of time at high rpm. Beyond that, .575 to .600 in. isn’t a problem with dual valve springs.

The stock rockers are good to 175 lbs. of seat pressure and 450 lbs. open. You will want to install a trunnion kit for added reliability.

There are a few other parts needed for a LH6 cam swap such as an LS2 timing chain, LS7 spec lifters, LS2 timing chain damper, and .080 in. wall pushrods.

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LH6 Power Adders

In general, the LH6 will see boost or nitrous before any serious head work. Before we get into the power adders, there are a couple things to address:

• A 4-corner steam kit reduces hot spots that cause the rings to butt and snap the piston’s ring lands.
• Any power-adder will put you well past the limitations of the stock injectors and pump. We’ll address those in the next section.

With that out of the way, let’s get to the fun stuff.

• A roots-style supercharger is dependable and makes great torque in the low- and mid-rpm range. It’s great for melting tires.
• A centrifugal-style supercharger is lightweight and makes more power at high rpm. This is partially due to a larger intercooler mounted in front of the radiator.
• A nitrous oxide kit (at low settings) is great for street driving with stock internals. Up to a 200 shot is common. Keep in mind the tight piston ring-gap is the limiting factor beyond that. If you’re wanting to get serious, a single plane intake is less prone to break from a nitrous backfire. A plate system has better distribution than the original intake, but an eight-nozzle fogger system is even better. Running higher octane fuel is advised.
• Truck engine bays make fitting turbos easy. Single turbo systems using turbo exhaust manifolds are an inexpensive way to make big power. If you’re running a single turbo, the T4 hot-side fits well, but the small turbine diameters limit exhaust flow. The 650 whp begins to feel like 400 did in a hurry, so take this into consideration. V-band style exhaust housings open up the turbine options and make plumbing easier. Although twins are a little more expensive out of the box, you’ll have more room to grow.

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Upgrading the LH6 Fuel System and Tuning

We recommend looking at the injector’s part number before taking it to the tuner.

The LH6 injectors were rated at 30 lbs. and won’t support much more than 430 hp at the wheels. Luckily, you have many options.

You will need an aftermarket injector that’s 1.890 in. between o-ring centers.

Keep in mind, the best injectors are fully characterized which helps your tuner maximize idle quality, etc.

When running boost, you can use a water-methanol system to supply extra fuel and lower charge air temps.

The factory pump is good to about 430 whp. Many fuel system upgrade options are available. Drop-in fuel pump modules and external pumps are popular. Other options to maintain or increase pump pressure includes electronic voltage controllers and hotwire kits.

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Upgrading the LH6 Intake Manifold and Throttle Body

If you have a power adder, the intake and throttle body can take a backseat for a while longer. If you are naturally aspirated though, it’s commonly done before the heads.

The factory truck-style manifold has long runners for better low-end torque. Porting the intake is one option and a good value.

In most cases a single plane is the wrong way to go, but might make sense if you’re running a lot of nitrous because it’s stronger and the cylinder-to-cylinder mixture distribution can be better.

If you’re looking for more power and torque, the F.A.S.T. LSXRT intake allows a bigger 102mm+ throttle body. Truck engine bays also accommodate tunnel rams. These trade a little bit of torque down low for more up high—and nothing looks cooler.

Ask your tuner about going with a Speed Density tune. Doing so removes the MAF restriction and will give you a bit more power.

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[Trying to find an LS engine for a swap or build? Check out Part 1 and Part 2 of our LS Spotter’s Guide.]

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Upgrading LH6 Cylinder Heads

• The stock heads can be CNC ported for more airflow and milled up to .030 in. for more compression. Flow numbers can be as high as 325 cfm at .600 lift. Lightweight hollow stem LS3 valves can be cut to 2.000 in. to fit the seats. Between the light valves and better springs, the engines will pull cleanly to 7000 rpm.
• A better option is aftermarket cathedral port heads. They reduce down time, they’re all new, and you can usually offset the added cost by selling your original heads. Valve angles are typically laid over to 13.5 degrees and 2.100 in. intake valves are common. They flow great and the cross-sections are great for boost. When comparing heads, look at .400 lift numbers as a general indicator of how the heads will perform. With a medium-sized cam, 425+ whp naturally aspirated is common even with the heavier truck drivetrain.

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Upgrading the LH6 Rotating Assembly

Still looking for more?

As mentioned before, the pistons are a weak point and you probably know a guy that’s popped one. A set of forged pistons should be high on your priority list. They have stronger wristpins, thicker ring lands, and the added valve reliefs allow you to run big cams.

There are exceptions, but Gen. 4 rods start getting dicey around 800 whp and the bolts don’t like much more than 7000 rpm. They are likely to bend before they break when subjected to real track conditions. If you’re getting forged pistons, it’s best to also get forged connecting rods with 7/16 rod bolts.

The LH6 cranks were cast but strong. They’ve been known to handle over 1000 whp. The main reason for going with a stroker forged crank is for the added cubic inches. The extra cubes bring boost on quicker which means you can use bigger turbos.

Performance rotating assemblies are available in many combinations.

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Upgrading the LH6 Engine Block

There is no replacement for displacement.

If you’re running boost, it’s common to overbore .020 in. to 3.800 in. The blocks can be taken to 3.905 (.007 oversize LS1) if it’s not going to see a lot of power.

When combined with a 4.000-inch stroke, this will increase displacement to 383 cubic inches.

The blocks have been known to withstand 850+ hp at the wheels with proper machining, racing fuel, and an excellent tune. Head and main studs are advised if you’re making more than 850 whp.

At that point, a set of head studs are a good idea. Although LS9 gaskets don’t have the optimal bore size, many have used them with success.

The factory main caps aren’t doweled. It’s better to reduce ignition timing and compensate with added boost to reduce the cylinder pressure spikes that lift heads and cause the main caps to dance.